1. Field of the Invention
This invention relates to an improved method of operating unthrottled internal combustion engines at compression ratios lower than required for diesel engines. Moreover, this invention relates to means for operating glow plugs in unthrottled engines at lower plug temperatures than would be required with non-catalytic glow plugs of the same size and geometry. In one specific embodiment, the plug temperature is provided with temperature determining means and electrical power is controlled to maintain the plug walls at a value determined by the engine speed and power output.
This invention also relates to catalytic glow plugs capable of igniting fuels at lower temperatures than a non-catalytic glow plug of the same size and shape.
2. Brief Description of the Prior Art
Existing diesel engines achieve a significantly higher thermal efficiency than conventional gasoline engines in automotive use and emit acceptable levels of carbon monoxide and light hydrocarbons. However, soot and nitrogen oxide levels are high and compression ratios are much higher than the optimum for maximum fuel economy. Moreover diesels are relatively hard to start as compared to automotive gasoline engines, even with electrically heated glow plugs, and require high cetane fuels. This is especially true of the lower compression diesels such as the large lower speed engines. With use of glow plugs, short plug life can be a problem particularly under operating conditions which require higher plug operating temperatures, such as cold starting at arctic temperatures.
As a means of improving cold starting performance of conventional high compression diesel engines with glow plugs, the use of catalytically self-heating glow plugs has been proposed (U.S. Pat No. 4,345,555). Such self-heating plugs are said to maintain a preset plug temperature by exothermic catalytic reactions after termination of the initial electrical heating of the plug during starting. A self-heating plug is said to maintain a higher temperature than a non-catalytic diesel glow plug and is further said to maintain a temperature above that required for ignition of fuel by a non-catalytic plug. It is taught that the catalyst should comprise a porous carrier, presumably to achieve greater surface heating (it is well known that such porous supports provide a greater surface area for catalytic reactions than a nonporous support). Self-heating plugs can be expected to offer no improvement in plug life as compared to conventional glow plugs inasmuch as such self-heating plugs are said to maintain a higher temperature than conventional plugs.
In addition to the above cited shortcomings, conventional diesels cannot be operated at low enough compression ratios for maximum efficiency and conventional diesels cannot efficiently utilize low cetane fuels such as methanol and gasoline. Although in-cylinder catalysts previously proposed can improve efficiency and reduce emissions of soot and nitrogen oxides, retrofitting of existing engines is not always economically feasible, especially with small automotive diesels.
Conventional spark ignition engines are typically less efficient than diesel engines in spite of operating in close approximation to the constant volume combustion Otto cycle, a more efficient cycle than the diesel cycle. This lower efficiency is believed to result primarily from the throttling losses associated with the requirement for spark ignition. Spark ignition requires near stoichiometric fuel-air mixtures for flame propagation. To control power levels, the amounts of fuel and air must both be varied in step. This requires throttling of the inlet air with resultant loss of pressure energy. Octane limits of fuels typically limit compression ratios to below optimum levels. Operation of spark engines without throttling of the inlet air could result in an engine more efficient than the diesel, even if such engines were limited to below optimum compression ratios.
Attempts have been made to operate unthrottled engines at lower than diesel compression ratios. With compression ratios too low for autoignition, an ignition source such as a spark plug or a continuously operating glow plug is needed. Thus, stratified charge spark-ignited engines of various designs, both piston and rotary, have been proposed. To date, such engines have not won acceptance. For use with heavy fuels such as diesel and jet A, spark plug fouling has been a severe problem leading to the use of glow plugs. Although use of glow plugs eliminates the fouling problem, a higher glow plug temperature is required for operating a low compression ratio engine than for cold starting a diesel engine. This is believed to be because the compression temperature of a low compression engine is lower than that of a high compression diesel at cold start conditions. Another factor is that the ignition temperature of hydrocarbon fuels may be higher at lower pressures than at higher pressures. With the high continuous operating temperature required using conventional glow plugs in a low compression engine, typically in excess of about 1375 degrees K., plug heat losses must be minimal if plug power requirements are to be acceptable at all operating conditions. With such a low heat loss plug it has been found that not only is no electrical power required at full load operation but that plug temperatures can even exceed the temperature limits of a high temperature material such as silicon nitride. Although much larger plugs could be used to lower operating temperature to some extent, power requirements would be excessive and space might not be available.
The method of the present invention overcomes the limitations of the prior art by providing glow plugs capable of ignition at a surface temperature as much as 300 degrees Kelvin lower than required for a non-catalytic glow plug of the same size and configuration and by providing an economical means of operating internal combustion engines at lower compression ratios without throttling of the inlet air and the throttling losses associated therewith. Use of the low ignition temperature catalytic glow plugs of the present invention in an internal combustion engine enables quicker starts inasmuch as less time is required to heat a plug to a lower temperature. Equally important, by providing a means of more rapid ignition at a lower plug temperature, combustion efficiency in engines is improved and emissions reduced. It is believed that the lower ignition temperature and more efficient combustion is a consequence of free radical production by the low porosity catalytic ignition surfaces of the present invention. It is known in the art that free radicals are combustion reaction intermediates.